This article points at several lists of alternatives 1 2.
Right after the announcement some people started working on a crowdsourced "Ning Alternatives" document using Google Docs which was started by a group of educators looking for alternatives to Ning but clearly lots of social networking startups contributed blurbs about their own stuff. Still it’s pretty useful. It also has sections on why educators like Ning and info about a petition to get Ning to keep free service for education/non-profits.
Several groups seems to be courting Ning users directly
